A demodulator circuit of the type named above is used for the demodulation of an amplitude-modulated signal emitted by a transmitter. One application are for example smart cards, in which a signal sent out by a so-called “reader” is received via a reception coil, and is subsequently evaluated by a demodulator circuit. In the course of this, high coil voltages must be reliably distinguished from low ones, and rising ones from falling ones.
A possibility for effecting this, which is known from the prior art consists in comparing the voltage that is present at the reception coil with a reference voltage, using a comparator. A comparator is essentially an operational amplifier that is operated not in the linear portion of the input/output characteristic, but predominantly in the saturation range, and therefore functions as a threshold switch module. If the coil voltage now exceeds the reference voltage, the comparator switches its output to “high”, and if the coil voltage lies below it, it switches to “low”. Depending on the wiring, inverting behavior can also be achieved.
Another known possibility is to evaluate the coil voltage not directly, but to evaluate its average value. One way of doing this is to provide a low-pass filter between the comparator and the coil. A further possibility is to evaluate an envelope of the coil voltage, and to provide a peak value rectifier for this. Finally, in place of the comparator one can also use an inverter or a so-called “Schmitt trigger”. Here, no external voltage reference is required, since simply the internal switching threshold, or else its envelope, is applied for the comparison with the coil voltage.
Due to various influencing factors, the progression of the coil voltage is not clearly defined: for example, the maximum value depends on the strength of the field emitted by the transmitter. The minimum value in turn depends on how much the transmitter can reduce the field during modulation. In the standard relevant for smart cards, ISO-14443-A, a maximum of 5% of the maximum value is envisaged for that, for example. Other influencing factors are the quality of the transmitting antenna as well as of the reception coil. All the aforementioned influencing factors can lead to the situation where the signal is “blurred”; in other words, during modulation the difference between the maximum and minimum values is reduced, as is the signal rise, and the detection in the demodulator circuit is thus made more difficult.
One thing that all the aforementioned demodulator circuits have in common is the disadvantage that the switching threshold is set at a fixed value, or in the case of the inverter or the Schmitt trigger it depends on the internal wiring of the component, and in addition it is dependent on the temperature and the supply voltage. These circuits are therefore suitable only to a limited degree for evaluating the signal forms mentioned above.
Besides the circuits already mentioned, from the prior art we also know of other design versions for demodulator circuits. For example, U.S. Pat. No. 6,636,146, “Contactless communication system for exchanging data” of 21 Oct. 2003, discloses a demodulator circuit that is particularly suitable for signals with a low modulation depth. Such signals are used for example in order to ensure a power supply to the receiver via the electromagnetic field emitted by the transmitter during the modulation phase too, i.e. when the transmitter reduces its transmitting power. Here, the demodulator circuit comprises a low-pass filter, with which the input signal is filtered. The output signal of the low-pass filter is supplied directly to a first connection of a comparator, and to the others via an RC combination that serves as an integrator and thus makes available the average value of the signal. Here, the comparator preferably has a hysteresis. The document also discloses a further variant of a demodulator circuit, which detects changes in the amplitude. Here, the input signal is once again supplied via a low-pass filter and its output signal is supplied via a differentiator stage to a first input of a comparator. The second input of the comparator is connected to a reference voltage, for example ground. Due to the method of construction, the demodulator circuit is insensitive to the level of the average value of the signal.
Patent US 2003/0128070, “A demodulator for an amplitude-modulated altering signal” of 10 Jul. 2003, furthermore discloses a circuit which comprises a peak value detector as well as two demodulators. One is provided for the purpose of detecting a signal peak and for generating an upper threshold value for recognizing the start of modulation, and a second one is provided for detecting a signal minimum and for generating a lower threshold value for recognizing the end of modulation. For this, the circuit comprises two comparators. A subsequent logic stage then produces the demodulated signal.
Patent US2003/0160650, “Wide dynamic range demodulator for smart cards or contactless tickets” of 28 Aug. 2003, furthermore discloses a demodulator circuit that has a peak value detector for producing the envelope of the input signal and a first module for displacing the received signal by the DC voltage portion of the signal. A further module produces a switching threshold for recognizing the start and end of the modulation. A comparator subsequently compares the two aforementioned signals and makes the demodulated signal available.
Finally, U.S. Pat. No. 5,930,304, “Wireless powered communication device with adaptive data detection and method” of 27 Jul. 1999, discloses a circuit in which a switching threshold of a comparator is altered depending on the strength of the signal received, in order to improve the demodulation.